This invention relates to a geochemical method of determining the location and shape of a uranium ore body. More specifically, the invention relates to determining the direction of offset drilling during the operation to define the uranium reserves of a roll-front type uranium deposit.
Uranium ore grade within a roll-front deposit is not uniform. Therefore, to evaluate the uranium reserves and plan an effective mining program, the distribution of ore grade must be delineated from gamma-ray data generated within bore holes which intersect the ore body. In plan view the roll-front assumes a sinuous pattern. Because of this shape the majority of the bore holes drilled in a reserve assessment program intersect the host sand either in the interior, behind the roll-front (updip) or in the exterior, in front of the roll-front (downdip) sediments associated with a roll-front body. When diagnostic characteristics of the interior or exterior sediments, i.e., color, gamma-ray configuration, are not definitive, the direction of offset is the result of geologic intuition. As a result, many holes may be offset in the wrong direction at high and wasted expenses.
The most commonly used technique of differentiating interior from exterior sediments is based on the color of the host sand. Within an actively migrating ore body the interior sands occur updip from the ore. Interior sands are buff colored, reflecting the dominance of ferric iron in the sediment. Uranium accumulates immediately downdip from the interface (also called the redox boundary) between the interior sediments and the exterior sediments. Exterior sands are grey in color, reflecting the dominance of ferrous iron. Thus during a reserve evaluation program, if the drill cuttings from the hosts and are buff in color, the next hole is offset downdip from the hole. Conversely, if the drill cuttings are grey, the next hole is offset updip.
The above method is simple and fast and requires no special training. However color of host sand cannot be used to discriminate the interior from exterior sands in all uranium deposits. For example basin tectonics, which occur following the emplacement of an ore body can superimpose reducing groundwaters upon the oxidizing sands. Consequently the color of the interior sand changes from buff to grey and the ore body is surrounded by grey sand. Another example such as a geologically rapid uplift, may cause a precipitous drop in the water table and strand an ore body. As a result the ore bodies are surrounded by buff sands.
Petrographic techniques of discerning interior from exterior sediments also exist. The techniques are based on the presence of certain mineral assemblages that retain their diagnostic integrity even after reducing conditions are superimposed upon the interior sediments. These minerals are not unique to a uranium deposit and several within the suite may occur in any aquifer which has a redox boundary. Thus drill cuttings from an overlying, non-uraniferous stratus can contaminate the sample and result in an erroneous classification of the sediment. In addition the sample must be examined shortly after collection or be dried and stored in a chemically and biologically inert environment to prevent changes in the mineral assemblage. This method also requires a relatively large amount of sample, 50 grams.